Single Silicon Wafer Micromachined Thermal Conduction Sensor

ABSTRACT

A single silicon wafer micromachined thermal conduction sensor is described. The sensor consists of a heat transfer cavity with a flat bottom and an arbitrary plane shape, which is created in a silicon substrate. A heated resistor with a temperature dependence resistance is deposed on a thin film bridge, which is the top of the cavity. A heat sink is the flat bottom of the cavity and parallel to the bridge completely. The heat transfer from the heated resistor to the heat sink is modulated by the change of the thermal conductivity of the gas or gas mixture filled in the cavity. This change can be measured to determine the composition concentration of the gas mixture or the pressure of the air in a vacuum system.

PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 14/045,555, filed on Oct. 3, 2013 in the name of the sameinventor and entitled “Single Silicon Wafer Micromachined ThermalConduction Sensor,” the disclosure of which is hereby incorporated intothe present application by reference.

FIELD

The invention relates to a thermal conduction sensor. Particularly, theinvention relates to a single silicon wafer micromachined thermalconduction sensor consisting of a heat transfer cavity having a flatbottom and an arbitrary plane shape, which are all created in a siliconsubstrate and in which no gas natural convection takes place and theheat transfer is through the thermal conduction of the gas filled in thecavity.

BACKGROUND

Gas sensors have been in use for several decades. The principle of mostcurrent gas sensors relies on chemical interactions between the gas anda specific material which provides high sensitivity. It is well knownthat such chemical sensors suffer from poor stability over time, due toinvolving chemical reactions. In order to circumvent this problemthermal conduction sensors that make only use of the physical propertiesof gases, have been developed.

Thermal conductivity measurement has been used in gas chromatography formore than 100 years, this gas detection method is still of largeimportance in process control and gas analysis. Furthermore, itpresently experiences a revival due to the progress in siliconmicromechanical electrical system (MEMS). A MEMS thermal conductionsensor allows a high degree of integration and miniaturization. Powerconsumption, response time and production costs of thermal conductionsensors can thus be dramatically reduced.

Ideally, the MEMS thermal conduction sensor consists of a heatedresistor, a heat sink and a gas cavity. The heated resistor is locatedon a thin film bridge and thus thermally insulated against a supportingsubstrate. The gas cavity is located between the heat source and theheat sink. When a MEMS thermal conduction sensor is in operation, heattransfers from the heat source to the heat sink. Any change in the gasconcentration, and thus the thermal conductivity of the gas cavity,results in a change in the bridge temperature. This change intemperature is measured with a temperature sensor, which is also locatedon the thin film bridge and is thermally linked very closely to theheated resistor.

Gerhard Pollak-Diener and E. Obermeier presented a micromachined thermalsensor which is suitable for the analysis of binary and ternary gasmixtures. Both the heat source and heat sink of the sensor are made ofsilicon. A first silicon wafer is etched in hot potassium hydroxide inorder to produce a thin film bridge for carrying a heated resistor. Asecond silicon wafer is etched to form a cavity and used as a heat sink.The second silicon wafer is mounted above the first silicon wafer. Thefirst silicon wafer is supported on the back side by an insulated wafer.(Heat-conduction micro-sensor based on silicon technology for theanalysis of two- and three-component gas mixtures, Sensors and ActuatorsB, 13-14 (1993) 345-347).

Arndt Michael and Lorenz Gerd revealed a thermal conduction sensorincluding a thermally insulated diaphragm formed by a recess in a baseplate exhibiting poor thermal conductivity. On one or both sides, thediaphragm is covered by a porous cover plate permitting gas exchange bydiffusion, a cavity being left open between the diaphragm and the porouscover plate. (U.S. Pat. No. 7,452,126, Arndt, et al., Nov. 18, 2008).

Several disadvantages can be found in the above first design. In thesensor fabrication process two silicon wafers are first processedindividually. Then the wafers are bonded together through a wafer-levelbonding process. Finally, the stack of the bonded wafer is mounted onthe surface of a insulate wafer. Such complicated and cumbersome processis far from efficient and economical.

In the above second design the fabrication process also consists ofwafer-level bonding. Furthermore, the silicon wafer is required to bondto a particular silicon carbine plate or an aluminum oxide plate insteadof a commonly used silicon wafer or a glass wafer.

In 2002 year, Isolde Simon and Michael Arndt reported a micromachinedconductivity sensor for hydrogen detection. A “hot” element of thesensor is realized as a platinum heat source structure on a thin filmsilicon-nitride dielectric bridge. A “cold” element is formed by thebulk silicon surrounding the bridge and by the gas surrounding thesensor. (Conductivity sensor for the detection of hydrogen in automotiveapplication, Sensors and Actuators, A97-98 (2002) 104-108)

In 2008 year, S. Udina et al discribed a thermal conduction sensor fornatural gas analysis. The sensor structure consists of a thin filmbridge defined on a silicon chip. The bridge is a multilayer sandwichstructure of silicon dioxide/silicon nitride and used as a hotplate. Apolysilicon heat source is located in the hotplate and a boron-dopedsilicon island is located right below as a thermal spreader for bettertemperature homogeneity across the hotplate. The backside of the die isattached to a metal casing (TO-8) acting as a heat sink to keep thesubstrate temperature approximately constant. The TO-8 casing presents adrilled hole in the area right below the bridge, which improves gasexchange with the surrounding atmosphere. (A micromachinedthermoelectric sensor for natural gas analysis: Thermal model andexperimental results, Sensors and Actuators B 134 (2008) 551-558).

In the above two design the heat generated by heat source flows throughthe surrounding gas instead of a gas cavity. In an open space it isimpossible to limit the heat transfer to be conduction, because theRayleigh number may be much higher than a critical number for naturalconvection to occur. The temperature behavior of the heater depends onboth conduction and natural convection, experimental data exploitationwill be quite tricky and difficult to analyze.

SUMMARY

In view of the above related arts, the present invention provides amicromachined thermal conduction sensor having the following features.

One feature of the micromachined thermal conduction sensor provided bythe present invention is that the sensor has a heat transfer cavityconsisting of a cavity, a thin film bridge carrying a heated resistorand a bottom used as a heat sink, which are all created in a siliconsubstrate such that wafer level bonding (is a packaging technology onwafer-level for the fabrication of microelectromechanical systems(MEMS)) is not required.

Another feature of the micromachined thermal conduction sensor providedby the present invention is that the cavity has a Grashof number (Gr isa dimensionless number in fluid dynamics and heat transfer whichapproximates the ratio of the buoyancy to viscous force acting on afluid) is less than 1×10⁻² such that no natural convection takes placecompletely in the cavity.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that the bottom of the cavity isflat and the gap between the thin film bridge and the bottom is uniformand consistency along the bottom.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that the thickness of the thin filmbridge is much less than the length of the bridge, which is beneficialfor the heat transfer from the heated resistor to the heat sink.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that the heated area by the heatedresistor is less than the area of the thin film bridge, which isbeneficial for the heat transfer from the heated resistor to the heatsink.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that the most fabrication processesof the sensor can be carried out in a stander COM production line, inwhich there are no wafer level bonding and double side alignmentavailable.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that the release of themicrostructure of the heat transfer cavity can be done at the final stepof the fabrication process.

Still another feature of the micromachined thermal conduction sensorprovided by the present invention is that without any changes the sensorcan be used as a vacuum sensor and a humidity sensor both based onthermal conductivity measurement.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiment(s) of the present invention will be understoodmore fully from the detailed description given below and from theaccompanying drawings of various embodiments of the invention, which,however, should not be taken to limit the invention to the specificembodiments, but are for explanation and understanding only.

FIG. 1 illustrates the perspective, partially cross-sectional,diagrammatic sketch of a single silicon wafer micromachined thermalconduction sensor in accordance with the present invention.

FIG. 2 to FIG. 5 illustrates the cross-sectional views of the singlesilicon wafer micromachined thermal conduction sensor of FIGS. 1 at 1 to4 stages in the fabrication thereof. The sensor is preferably fabricatedusing standard silicon processing techniques (photolithography, Thermaldiffusion, LPCVD, wet and dry etching etc.).

FIG. 6 to FIG. 7A illustrates the top-plane views of the single siliconwafer micromachined thermal conduction sensor of FIGS. 1 at 6 to 7stages in the fabrication thereof.

FIG.7B to FIG. 8 illustrates the cross-sectional views of the singlesilicon wafer micromachined thermal conduction sensor of FIGS. 1 at 8 to9 stages in the fabrication thereof.

DETAILED DESCRIPTION

In order to achieve the above objects, referring to FIG. 1, a singlesilicon wafer micromachined thermal conduction sensor consists of a heattransfer cavity 102 having a flat bottom and an arbitrary plane shape,which is all created in a silicon substrate 101 and in which heattransfer is through gas thermal conduction in one direction, as shown byarrows 109, a frame 103 resulted by creating the cavity 102 and having acurved wall supporting and surrounding the cavity 102, a thin filmbridge 104 suspending over the cavity 102 and having a central widesection and two side narrow sections 105 on the two opposite sides ofthe central wide section, which is arranged in a line and extending tothe edge of the frame 103, a heated resistor 108 disposed on the surfaceof the central wide section of the bridge 104, a heat sink 106 being theflat bottom of the cavity 102 and parallel to the bridge 104, atemperature sensors 111 disposed on the surface of the frame 103, atleast one openings 107 disposed between the bridge 104 and frame 103 andused for the cavity 102 to communicate with the outside, four bondingpads 110 and 112 are disposed near the edge of the surface of thesubstrate 101, which electrically connect the heated resistor 108 andthe temperature sensor 111 to an outside circuit.

It should be emphasized that the gap of the heat transfer cavity 102 isrestricted in the range of 5 to 50 microns. In this case no naturalconvection takes place completely in the cavity. The heat transfer fromthe heated resistor 108 to the heat sink 106 is through the gas thermalconduction in the cavity along the vertical direction, as shown byarrows 109 in FIG.1.

It should be understood that in fluid dynamics and heat transfer Grashofnumber approximates the ratio of the buoyancy to viscous force acting ona fluid. For an enclosed cavity Grashof number can be expressed byG_(rL)=gβ(T_(h)−Tc)L³/v², where L=cell depth, g=acceleration due togravity, T_(h)=temperature of heat source, T_(c)=temperature of heatsink, v=Kinematic viscosity (for air, v=20.94×10⁻⁶ (m²/s), β=Thermalexpansion coefficient (for air, β=2.83×10⁻³). Assuming that the cavity102 is filled with air, the temperature different between the heatedresistor 108 and the heat sink 106 is 200° C., the gap of the cavity is50 microns, it can be calculated that the Grashof number is 1.581×10⁻³.Such small Grashof number means that in the cavity the heat transfer ofone dimensional conduction overwhelmingly prevails over naturalconvection.

It is necessary to be noted that the heat transfer cavity 102 may takean arbitrary plane shape. It is preferred to be square or circle, butnot limited to these two shapes. The side length of the square shape maybe chosen in the range from 400 to 2000 microns. The diameter of thecircle shape may be chosen in the range from 400 to 2000 microns.

It is preferred that the bridge 104 consists of SiO₂/Si₃N₄ doublelayers. As well known, a silicon nitride layer and a silicon oxide layerdeposited on a silicon wafer possess compressive and tensile intrinsicstresses, respectively. The combination of nitride/oxide double layerswith the opposite stresses led to a lower residual tensile stress.

As an alternative, the bridge 104 consists of Si₃N₄/polysilicon/Si₃N₄sandwiched layers. The residual stress in an as-LPCVD-depositedpolysilicon layer is compressive, which can be used to offset thetensile stress of the silicon nitride layer. In addition, thepolysilicon layer sandwiched between the two silicon nitride layers mayuniform the temperature distribution of the bridge 104, becausepolysilicon possess higher a thermal conductivity than silicon nitride.

Preferably, the bridge 104 has a central wide section and two sidenarrow sections, which are arranged in a line and extend to the edge ofthe frame 103 and has a length ranging from 200 to 800 microns and athickness ranging from 1 to 3 microns.

Preferably, the heated resistor 108 and the temperature sensor 110 aremade of Nickel or Platinum in order to obtain a temperature dependenceresistance. The heated resistor 108 is disposed on the surface of thecentral wide section of the bridge 104 and may be arranged in zigzagshape in order to obtain an appropriate resistance. The temperaturesensor 110 is disposed on the surface of the silicon frame 103 and mayalso be arranged in zigzag shape.

The goal of the design of the thermal conduction sensor is to simply thetemperature behavior of the heated resistor such that it may bedescribed by one dimensional mathematical model. In this model, thefollowing assumptions should be true: the bridge 104 carrying the heatedresistor 108 and the heat sink 107 are parallel, and the thickness ofthe bridge 104 is much smaller than its length; all the heat generatedby the heated resistor 108 is transferred to the heat sink 107 bythermal conduction through the gas filled in the cavity 102; the gap ofthe cavity 102 and the temperature of the heated area by the resistor108 are uniform and consistency. Based on one dimensional mathematicalmodel, the thermal conduction sensor can be used in the analysis ofmixtures gas composition, such as natural gas consisting of methane,hydrocarbons, carbon dioxide and nitrogen. Obviously, the thermalconduction sensor can also be used as a thermal conductivity vacuumsensor and a thermal conductivity humidity sensor without any changes.

Referring in more detail to FIG. 2 through FIG. 8, the method offabricating the single silicon wafer micromachined thermal conductionsensor in accordance with the present invention will be described.

The thermal conduction sensor is fabricated using a silicon substrate asa starting material. The crystal orientation and the resistivity of thesilicon substrate are p⁺-type (100) and in the range of 0.1 to 0.001ohm-cm, respectively.

As shown in FIG. 2, the first fabrication step of the sensor is todeposit a silicon nitride layer on the surface of the substrate 201 byLPCVD (low pressure chemical vapor deposition). The thickness of thesilicon nitride layer is chosen in the range between 1500 to 3000Angstroms. A photolithography process is performed to create ananodization mask 202 through partially etching the silicon nitridelayer. The etching of the silicon nitride is carried out by dry etching.A resulted transparent window in the silicon nitride layer is a squareshape with a side length ranging from 400 to 2000 microns. It should benoted that the resulted transparent window is not limited to squareshape. It may be an arbitrary shape including a circle shape with adiameter ranging from 400 to 2000 microns.

The second fabrication step is to conduct the anodization of the siliconsubstrate 201 in a HF solution, as shown in FIG. 3. An anodization cellto be used is made of Teflon and divided into two compartments by thesilicon substrate to be anodized. Each compartment has a platinumelectrode with its distal end connecting to a power supply. A HFsolution to be used is a mixer of 1 or 2 part 48 wt % HF and 1 partethanol. The anodic current is chosen in the range from 40 to 80mA/cm.sup.2, which is kept constant during the anodization. The formedporous silicon has a porosity of about 50 to 70%. It can be seen thatthe anodization is restricted in the square region and only to convertthe upper layer of the silicon substrate 201 into porous silicon layer203, which has a thickness in the range of 5 to 50 microns. The poroussilicon layer 203 is subjected to a post thermal treatment forstabilizing porous structure. The thermal treatment is performed at400.degree Celsius in dry oxygen for 30 min. As a result, one or twoatom layer thick silicon dioxide film is formed on the surface of theinner pores of the porous silicon, which can stabilize the porousstructure of the porous silicon layer 203.

After removing the residual silicon nitride layer the third fabricationstep is to follow. This step is to deposit a first insulating layer 204consisting of silicon nitride on the surface of the silicon substrate201 including the surface of the porous silicon layer 203, as shown inFIG. 4. The thickness of the silicon nitride layer is in the range of1500 to 3000 Angstroms. The deposition of silicon nitride is conductedby LPCVD process. Clearly, the insulating film 205 can be used to sealthe pores of the porous silicon layer 203 so as to prevent the poroussilicon layer from degenerate.

As shown in FIG. 5, the fourth fabrication step is for creating apolysilicon block 205. To do this, a polysilicon layer is firstdeposited on the surface of the insulating layer 204 by LPCVD. Thethickness of the polysilicon layer may be 1 to 4 microns. After aphotolithography process a part of the polysilicon layer is etched awayby dry etching. As a result, the polysilicon block 205 is formed,beneath which there is the porous silicon layer 203.

The fifth fabrication step is to deposit a second insulating layer 206consisting of silicon nitride layer on the surface of the substrate 201including the surface of the polysilicon block 205 by LPCVD. Thethickness of the insulating film 206 is in the range from 1500 to 3000Angstroms. It can be seen that the second insulating film 206 combinesthe first insulating layer 204 to form a composite insulating layeraround the polysilicon block 205.

The sixth fabrication step is to build a heated resistor 207, atemperature sensor 209 and four bonding pads 208 and 210 on the surfaceof the second insulating layer 206. It is preferred that the heatedresistor 207 and temperature sensor 209 are made of platinum and formedby a lift-off process. The lift-off process consists of the followingsteps. First, a pattern of positive photoresist is created by aphotoresist process. Secondly, a platinum layer is deposited over thephotoresist pattern by electron beam evaporation or sputtering. Thirdly,the pattern of positive photoresist is removed by immerse in acetonesolution. As shown in FIG. 6 the resulted platinum resistors arearranged in zigzag shape. The thickness of the platinum layer is in therange of 1000 to 3000 Angstroms and the resistance of the platinumresistors is in the range of 50 to 1000 ohm.

As an alternative, the heated resistor 207 and temperature sensor 209are made of nickel. Since nickel can be etched by wet etching nickelpattern can be created on the surface of the silicon substrate 201without using lift-off process.

It should be noted that the silicon substrate with the photoresistpattern is not be allowed to heat beyond 150.degree censes during thelift-off process. This is to ensure that the photoresist is not harmedso that it is easy to remove by immersion in acetone. The lift-offprocess results a pattern consisting of a heated resistor 207, twobonding pads 208 for the heated resistor 207, a temperature sensor 209,and two bonding pads 210 for the temperature sensor 209, as shown inFIG. 5. Generally, the platinum pattern deposited by sputtering needs tobe annealing at 500 degree censes in nitrogen for 30 min beforeperforming the next fabrication step.

As shown in FIG. 7B, the seventh fabrication step is to form 4000Angstroms thick amorphous silicon carbide film 213 protecting thesurface of the substrate including the surface of the platinum patternby PECVD. Then a photolithography process is performed to reveal thebonding pads 208 and 210. The unwanted amorphous silicon carbide isremoved by dry etching.

As shown in FIG. 7A, the eighth fabrication step is to create openings214 in the first insulating film 204 and the second insulating film 206around the polysilicon block 205. This is done by forming a photoresistpattern and then dry etching the insulating film using the photoresistpattern as a etch mask.

The ninth fabrication step is to etch away the porous silicon in theopenings and under the polysilicon block. The etchant to be used is aKOH solution of 1-3 w %. The etchant cannot attack the porous siliconlayer immediately since the surface of the inner pores of the poroussilicon is coated with a thin silicon nitride film or silicon dioxidefilm. The etchant also cannot attack the silicon of the siliconsubstrate since the KOH solution is diluted and the etching is performedat room temperature. After the etching, a heat transfer cavity 215, athin film bridge 216 being the top of the cavity 215, four side narrowsections 217 supporting the central wide section of the bridge 216, aframe 218 surrounding the cavity 215, and a heat sink 219 being the flatbottom of the cavity 215 are formed, as shown in FIG. 8.

Other embodiments are within the spirit and scope of the appendedclaims. Numerous variations and alternate embodiments will occur tothose skilled in the art without departing from the spirit and scope ofthe invention.

While the present invention has been described with reference toparticular embodiment(s) of the silicon wafer micromachined thermalconduction sensor, it is obvious that other embodiments can be usedwithout departing from the teachings herein. Obviously, manymodifications and variations are possible in light of the teachingherein. It is therefore to be understood that within the scope of theappended claims, the present invention may be practiced other than asspecifically described.

What is claimed is:
 1. A method of manufacturing a single silicon wafermicromechanical thermal conduction sensor comprising steps of: providinga silicon substrate having a resistivity ranging from 0.1 to 0.001ohm-cm and a (100) crystal orientation; depositing a silicon nitridelayer on the surface of the silicon substrate by LPCVD (low pressurechemical vapor deposition) which has a thickness in the range of 2000 to3000 Angstroms; performing first lithographic process including forminga photoresist pattern with a square or a circles shape window in thesilicon nitride layer and etching the exposed silicon nitride layer inthe window; performing anodization in a HF solution to convert theexposed single crystal silicon layer of the silicon substrate into aporous silicon layer; depositing first insulating silicon nitride layeron the surface of the silicon substrate including the surface of theporous silicon layer by LPCVD; depositing a polysilicon layer on thesurface of the first insulating silicon nitride layer by LPCVD;performing second photolithography process for creating a polysiliconpattern on the top central region of the porous silicon layer;depositing second insulating silicon nitride layer coating the surfaceof the silicon substrate including the surface of the polysiliconpattern by LPCVD; creating a heated resistor on the top surface of thepolysilicon pattern and a temperature sensor on the periphery topsurface of the porous silicon layer by a lift-off process includingthird photolithography process and metal deposition by E-beamevaporation or sputtering; depositing a passivation layer over thesurface of the silicon substrate by LPCVD or PECVD (plasma enhancedchemical vapor deposition); performing fourth lithographic process toreveal the bonding pads; performing fifth lithographic process to createat least one opening in the insulating layer so as to reveal it'sbeneath porous silicon layer; and etching the porous silicon layerthrough the openings in a diluted HF solution so as to form a heattransfer cavity having a flat bottom parallel to the surface ofsubstrate, a frame having a side curved wall surrounding the cavity, athin film bridge suspending over the cavity and having a central sectionand at least two side sections on the two opposite sides of the centralsection, which extend to the edge of the frame.
 2. The method ofmanufacturing a single silicon wafer micromechanical thermal conductionsensor as cited in claim 9, wherein said porous silicon layer has athickness ranging from 5 to 50 microns and is formed under thecondition: HF solution consisting of one or two parts 48 wt % HF and 1part ethanol; anodic current density 40 to 80 mA/cm.sup.2.
 3. The methodof manufacturing a single silicon wafer micromechanical thermalconduction sensor as cited in claim 9, wherein said heat transfer cavityhas a square plane shape with a side length ranging from 400 to 2000microns and a vertical gap ranging from 5 to 50 microns.
 4. The methodof manufacturing a single silicon wafer micromechanical thermalconduction sensor as cited in claim 9, wherein said heat transfer cavityhas a circle plane shape with a diameter ranging from 400 to 2000microns and a vertical gap ranging from 5 to 50 microns.
 5. The methodof manufacturing a single silicon wafer micromechanical thermalconduction sensor as cited in claim 9, wherein said side narrow sectionsof the thin film bridge are made SiO₂/Si₃N₄ and have a beam shape with alength ranging from 200 to 800 microns and a thickness ranging from 1 to2 microns.
 6. The method of manufacturing a single silicon wafermicromechanical thermal conduction sensor as cited in claim 9, whereinsaid wide central section of the thin film bridge is made ofSi₃N₄/polysilicon/Si₃N₄ and has a square shape with a side lengthranging from 200 to 1000 microns and a thickness ranging from 1 to 3microns.
 7. The method of manufacturing the single silicon wafermicromachined thermal conduction sensor as recited in claim 9, whereinsaid heated resistor is made of Nickel or Platinum and has a resistanceranging from 50 to 5000 ohm.
 8. The method of manufacturing the singlesilicon wafer micromachined thermal conduction sensor as recited inclaim 9, wherein said temperature sensor is made of Nickel or Platinumand has a resistance ranging from 50 to 5000 ohm.